1. Field of the Invention
The present invention relates to a slot antenna apparatus for transmitting and receiving analog radio-frequency signals or digital signals in a microwave band, a millimeter-wave band, etc. More particularly, the present invention relates to a slot antenna apparatus that eliminates unstable radiation due to its grounding structure.
2. Description of the Related Art
A wireless device operable in a much wider band than that of prior art devices is required for the following two reasons. As the first reason, it is intended to implement a novel short-range wireless communication system with the authorization of use of a very wide frequency band, i.e., an ultra-wideband (UWB) wireless communication system. As the second reason, it is intended to utilize a variety of communication systems each using different frequencies, by means of one terminal.
For example, when converting a frequency band into a fractional bandwidth being normalized by a center frequency “f0” of an operating band, a frequency band from 3.1 GHz to 10.6 GHz authorized for UWB in U.S.A. corresponds to a value of 109.5%, indicating a very wide band. On the other hand, in cases of a patch antenna and a one-half effective wavelength slot antenna which are known as basic antennas, the operating bands converted to fractional bandwidths are less than 5% and less than 10%, respectively, and thus, such antennas cannot achieve a wideband property such as that of UWB. For example, referring to the frequency bands currently used for wireless communications in the world, a fractional bandwidth to the extent of 30% should be achieved in order to cover bands from the 1.8 GHz band to the 2.4 GHz band with one same antenna, and similarly, a fractional bandwidth to the extent of 90% should be achieved in order to simultaneously cover the 800 MHz band and the 2 GHz band with one same antenna. Furthermore, in order to simultaneously cover bands from the 800 MHz band to the 2.4 GHz band, a fractional bandwidth of 100% or more is required. The more the number of systems simultaneously handled by one same terminal increases, thus resulting in the extension of a frequency band to be covered, the more a wideband antenna with small size is required to be implemented.
Moreover, it is considered to apply a balanced line with high noise immunity and operable in a low voltage, to a feed line of an antenna designed for a high-speed communication system, and to transmission lines for use in a circuit of high-frequency devices. While a conventional unbalanced line is formed of a planar grounding conductor and one strip-shaped signal line conductor, a balanced line is formed of a planar grounding conductor and two parallel strip-shaped signal line conductors. In the balanced line, a signal is transmitted as a potential difference between two signal lines provided in one same plane on a dielectric substrate, thus requiring a specific structure and circuit of input and output terminals. In order to design high-frequency devices suitable for high-speed communication systems, a balanced line can be applied to a feed line of an antenna, to active devices connected to feed lines in use, such as antenna switches or amplifiers, or to passive devices, such as bandpass filters.
A one-end-opened one-quarter effective wavelength slot antenna is one of the most basic planar antennas, and a schematic view of this antenna is shown in FIGS. 34A, 34B, and 34C (hereinafter, referred to as a “first prior art example”). FIG. 34A is a schematic top view showing a structure of a typical one-quarter effective wavelength slot antenna (showing a grounding conductor 103 on a backside in phantom view), FIG. 34B is a schematic cross-sectional view of the slot antenna in FIG. 34A, and FIG. 34C is a schematic view showing a backside structure of the slot antenna in FIG. 34A in phantom view. As shown in FIGS. 34A, 34B, and 34C, a feed line 113 is provided on a front-side of a dielectric substrate 101, and a notch with a width “Ws” and a length “Ls” is formed in a depth direction 109a from an outer edge 105a of an infinite grounding conductor 103 provided on a backside thereof. The notch operates as a slot resonator 111, one of its ends is opened at an open end 107. The slot 111 is a circuit element which is obtained by completely removing a conductor in thickness direction, in a partial region of the grounding conductor 103, and which resonates near a frequency “fs” at which one-quarter of the effective wavelength is equivalent to the slot length “Ls”. The feed line 113 formed in a width direction 109b intersects with the slot 111 at a portion thereof, and electromagnetically excites the slot 111. A connection to an external circuit is established through an input terminal. It is noted that according to common practice, a distance “Lm” of the feed line 113 from its open-ended termination point 119 to the slot 111 is set to the extent of one-quarter effective wavelength at the frequency “fs”, so as to achieve input impedance matching. Further, it is noted that according to common practice, a line width “W1” is designed based on a thickness “H” of the substrate and a permittivity of the substrate, such that the characteristic impedance of the feed line 113 is set to 50Ω.
As shown in FIGS. 35A, 35B, and 35C, Patent Document 1 discloses a structure for operating the one-quarter effective wavelength slot antenna shown in the first prior art example, at a plurality of resonant frequencies (hereinafter, referred to as a “second prior art example”). A slot 111 has a slot length “Ls”, and includes a capacitor 16 so as to connect points 16a and 16b each located a distance “Ls2” away from an open end. When the antenna is excited at a plurality of resonant frequencies at a feeding point 15, the antenna operates with different slot lengths “Ls” and “Ls2” as shown in FIGS. 35B and 35C, and thus the bandwidth can be extended. However, according to the frequency characteristics shown in Patent Document 1, it is not enough to obtain a currently required ultra-wideband characteristics.
Non-Patent Document 1 discloses a method of operating a slot resonator in a wideband, which is short-circuited at both ends of a slot, and is of a one-half effective wavelength slot antenna (hereinafter, referred to as the “third prior art example”). FIG. 36 is a schematic top view showing a structure of a slot antenna described in Non-Patent Document 1. Referring to FIG. 36, a grounding conductor 103 and a slot 111 on a backside of a substrate are shown in phantom view. The slot 111 is formed in the grounding conductor 103, such that the slot 111 has a certain width “Ws”, and a length “Ls” equivalent to one-half effective wavelength, and such that the slot 111 is coupled to a feed line 113 at a position 51a which is offset by a distance “d” from the center of the slot 111. According to prior art methods for matching input impedance of a slot antenna, a method has been used in which for exciting the slot 111, the feed line 113 intersects with the slot 111 at a position on the feed line 113 apart from an open-ended termination point 119 by one-quarter effective wavelength at a frequency “fs”. However, as shown in FIG. 36, in the third prior art example, a region extending over a distance “Lind” from the open-ended termination point 119 of the feed line 113 is replaced by an inductive region 121 which is a transmission line with a characteristic impedance higher than 50Ω, and that inductive region 121 is coupled to the slot 111 at substantially the center of the inductive region 121 (i.e., in FIG. 36, “t1” and “t2” are substantially equal to each other). In this case, a width “W2” of the inductive region 121 is set to a certain width narrower than the width of the feed line 113, the length “Lind” of the inductive region 121 is set to one-quarter effective wavelength at a center frequency “f0” of an operating band, and the inductive region 121 operates as a one-quarter wavelength resonator different from the slot resonator. As a result, an equivalent circuit structure includes two resonators, which is increased from one resonator that is included in a typical slot antenna, and a double-resonance operation is achieved by coupling the resonators resonating at frequencies close to each other. In an example shown in FIG. 2(b) of Non-Patent Document 1, a good reflection impedance characteristic of −10 dB or less is achieved at a fractional bandwidth of 32% (near 4.1 GHz to near 5.7 GHz). As shown in comparison of actual measurement results of reflection characteristics versus frequency in FIG. 4 of Non-Patent Document 1, the fractional bandwidth of the antenna of the third prior art example is much wider than a fractional bandwidth of 9% of a typical slot antenna fabricated under conditions using the same substrate.
FIG. 37 is a schematic view showing a method for measuring a mobile phone antenna described in Non-Patent Document 2 (hereinafter, referred to as the “fourth prior art example”). When measuring a mobile phone 2 under test by a network analyzer 1, in conventional technique, they are connected through a radio-frequency (RF) unbalanced feed circuit, such as a radio-frequency cable. However, Non-Patent Document 2 reported that when using an unbalanced feed circuit to feed a small-sized communication terminal having a grounding conductor of a finite area available for antenna operation, an unbalanced grounding conductor current occurring in the grounding conductor flows back into a grounding conductor of a feed circuit in a measuring apparatus, thus affecting the measurement accuracy itself of radiation characteristics and impedance characteristics. Hence, as shown in FIG. 37, Non-Patent Document 2 discloses that instead of feeding by using a radio-frequency unbalanced feed circuit, a photodiode (PD) 2a and a light-emitting diode (LD) 2c are provided in the mobile phone 2 as an input terminal and an output terminal, and further, a light-emitting diode 4 and a photodiode 5 are provided also in the network analyzer 1, and they are connected by optical fibers (shown by dotted lines in FIG. 37). A signal S1 outputted from the network analyzer 1, and a signal S2 reflected from a feeding point S3 of an antenna 3 and inputted to the network analyzer 1 are transmitted by different optical fibers. An inputted wave and a reflected wave to/from the antenna 3 are separated by a circulator 2b. The use of optical fibers upon feeding enables to isolate a grounding conductor from a feed system in the mobile phone 2, thus achieving a measurement without adverse effects of an unbalanced grounding conductor current in a small-sized antenna.
Prior art documents related to the present invention are as follows:
(1) Patent Document 1: Japanese Patent laid-open Publication No. 2004-336328;
(2) Non-Patent Document 1: L. Zhu, et al., “A Novel Broadband Microstrip-Fed Wide Slot Antenna With Double Rejection Zeros”, IEEE Antennas and Wireless Propagation Letters, Vol. 2, pp. 194-196, 2003; and
(3) Non-Patent Document 2: Fukazawa, et al., FUKAZAWA et al., “Impedance Measurement of the Antenna on the Portable Telephone using Fiber-Optics”, Proceedings of the 2003 IEICE (The Institute of Electronics, Information and Communication Engineers) General Conference, B-1-206, p. 206, 2003.
As discussed above, sufficient wide band operation has not been achieved in the prior art slot antennas. Additionally, even if the wideband property can be achieved with a small-sized configuration, radiation characteristics and input impedance characteristics are unstable depending on a connection between an antenna and an external unbalanced feed circuit. Thus, it is hard to determine characteristics to be exhibited when the antenna is mounted on a wireless communication terminal apparatus.
First of all, in the case of the typical one-end-opened slot antenna with only one resonator in its configuration as in the first prior art example, the antenna can operate in a resonant mode within only a limited band, and thus, a frequency band, where a good reflection impedance characteristic can be achieved, is limited to a fractional bandwidth to the extent of a little less than 10%.
In the second prior art example, although a wideband operation is achieved by incorporating the capacitive reactance element into the slot, it can be readily noticed that additional components such as the chip capacitor are required, and the characteristics of the antenna vary depending on variations in characteristics of the newly incorporated additional components. Further, according to the examples disclosed in FIGS. 14 and 18 of Patent Document 1, it is hard to achieve characteristics of input impedance matching with low reflection across an ultra-wideband.
In the third prior art example, the fractional bandwidth characteristic is limited to the extent of 35%. Further, as compared to the antennas of the first and second prior art examples with one-end-opened slot resonators which are of one-quarter effective wavelength resonators, it is disadvantageous in reducing size to use the slot resonator which is short-circuited at both ends and is of the one-half effective wavelength resonator.
Accordingly, even if incorporating the principle of the double-resonance operation according to the third prior art example when designing the one-quarter effective wavelength slot antenna according to the first or second prior art example, the unbalanced grounding conductor current flows back into the grounding conductor of the unbalanced feed circuit connected to the antenna during the antenna operation, as pointed out in Non-Patent Document 2. The radiation characteristics and input impedance characteristics of the antenna vary depending on the shape of the unbalanced feed circuit through which the unbalanced grounding conductor current flows, for example, depending on a length of a coaxial cable which is connected to the antenna to determine the characteristics. Particularly, the radiation characteristics severely vary depending on the conditions of an external circuit.